sizeof（value）vs sizeof（type）？ [英] sizeof(value) vs sizeof(type)?

问题描述

考虑：

double data; 
double array[10]; 
std::vector<int> vec(4, 100); 
MyClass myclass;   

之间是否存在差异：

sizeof(double);
sizeof(double[10]);
sizeof(std::vector<int>);
sizeof(MyClass);

和

sizeof(data);
sizeof(array);
sizeof(vec);
sizeof(myclass);

这两种语法是否不同或严格等效？是否在编译时都对它们进行了评估？如果不是，则在运行时评估哪个？

Are the two syntaxes different or strictly equivalent ? Are all of them evaluated at compile-time ? If not, which one is evaluated at run-time ?

推荐答案

唯一的区别在于语法和方便性。

The only differences are in syntax and convenience.

从语法上讲，在一种情况下，您可以省略括号，但在另一种情况下则不允许：

Syntactically, you're allowed to leave out the parentheses in one case, but not the other:

double d;

sizeof(double); // compiles
sizeof(d);      // compiles
sizeof d;       // compiles
sizeof double;  // does NOT compile

就方便程度而言，请考虑以下内容：

As far as convenience goes, consider something like:

float a;

x = sizeof(float);

y = sizeof(a);

例如，如果某个时候您最终更改了 a 从 float 变为双精度，您还需要将 sizeof（float）更改为 sizeof（double）进行匹配。如果您始终使用 sizeof（a），则当您从 float更改 a 时， code>到 double ，您对 sizeof 的所有使用也会自动更改，而无需进行任何编辑。后者比C ++在C语言中更常见，例如在调用malloc时：

If, for example, you sometime end up changing a from a float to a double, you'd also need to change sizeof(float) to sizeof(double) to match. If you use sizeof(a) throughout, when you change a from a float to a double, all your uses of sizeof will automatically change too, without any editing. The latter is more often a problem in C than C++, such as when calling malloc:

float *a = malloc(10 * sizeof(float));

vs。

float *a = malloc(10 * sizeof(*a));

在第一种情况下，更改第一个 float 到 double 将产生可编译的代码，但有缓冲区溢出。在第二种情况下，将（仅） float 更改为 double 可以正常工作。

In the first case, changing the first float to double will produce code that compiles, but has a buffer overrun. In the second case, changing the (only) float to double works fine.

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